Photonic integrated circuits hold the potential of creating low cost, compact optical functions. The application fields in which they can be applied are very diverse: telecommunication and data communication applications, sensing, signal processing, etc. These optical circuits comprise different optical elements such as light sources, optical modulators, spatial switches, optical filters, photodetectors, etc., the optical elements being interconnected by optical waveguides. Silicon photonics is emerging as one of the most promising technologies for low cost integrated circuits for optical communication systems. It is CMOS-compatible and due to the available high refractive index contrast, it is possible to create very compact devices.
However, coupling of light between an optical element such as for example an optical fiber and an optical waveguide, e.g. an optical waveguide on a silicon chip, is challenging because of the large mismatch in mode-size between the integrated nanophotonic waveguides (typically 0.1 μm2) and standard single mode fibers (typically 100 μm2). This may lead to high coupling losses, for example in the order of 20 dB. Therefore, there is a great interest in improving the coupling efficiency between an optical waveguide circuit and an optical fiber or more in general for improving the coupling efficiency between an integrated optical waveguide and an optical element (e.g. light source, modulator, optical amplifier, photodetector) or between an integrated optical waveguide and free space.
Different technologies are presented in the literature to enhance the coupling efficiency between an integrated optical waveguide and an optical fiber. One possible solution is a lateral coupler using spot size conversion with an inverse taper, in combination with a tapered or lensed optical fiber. Although this technique allows broadband and polarization independent coupling, the 1 dB alignment tolerances are very small (typically about 0.3 μm). Moreover, this approach requires cleaved and polished facets to couple light into the optical circuit. This excludes its use for wafer-scale optical testing of the optical functions, and may lead to a high cost.
In order to improve the coupling efficiency to a standard single mode fiber in a high refractive index contrast system, and in order to relax the alignment accuracy of an optical fiber and to allow for wafer scale testing, one-dimensional grating structures have been proposed. These structures allow direct physical abutment from the top or bottom side of the optical waveguide circuit with a standard single mode optical fiber, while the diffraction grating directs the light into the optical waveguide circuit (or vice versa). The performance of these one-dimensional gratings is critically dependent on the polarization of the light. Typically, only a single polarization at a certain wavelength can be efficiently coupled between the integrated optical waveguide and an optical fiber, resulting in a very polarization dependent operation of the one-dimensional grating coupler. As in typical applications this polarization is unknown and varying over time, the applicability of the one-dimensional grating structures may be limited. In cases where a polarization maintaining fiber is used or where a polarization scrambling approach is adopted, these one-dimensional gratings can be used. Also in the case where the one-dimensional grating structure is used to optically couple an integrated light source, generating, processing or detecting light with a known and fixed polarization, these devices can be used.
In order to be able to handle situations where polarization is unknown and varying over time, a two-dimensional grating coupler structure has been proposed (U.S. Pat. No. 7,065,272), which comprises two optical waveguides intersecting at a substantially right angle and a two-dimensional diffractive grating structure created at the intersection. When the diffractive grating is physically abutted with a single mode optical fiber, a polarization split is obtained that couples orthogonal modes from the single-mode optical fiber into identical modes in the first and second waveguide. While the ratio of coupled optical power between both optical waveguides is still dependent on the polarization of the incident light, this two-dimensional fiber coupling structure can be used in a polarization diversity approach, in order to achieve a polarization independent integrated circuit, i.e. an integrated circuit wherein the processing of the radiation is performed independent of the polarization of the incoming radiation.